Absolute dimensions of detached eclipsing binaries - III. The metallic-lined system YZ Cassiopeiae

The bright binary system YZ Cassiopeiae is a remarkable laboratory for studying the Am phenomenon. It consists of a metallic-lined A2 star and an F2 dwarf on a circular orbit, which undergo total and annular eclipses. We present an analysis of 15 published light curves and 42 new high-quality échelle spectra, resulting in measurements of the masses, radii, effective temperatures and photospheric chemical abundances of the two stars. The masses and radii are measured to 0.5 per cent precision: M_A = 2.263 ± 0.012 M_⊙, M_B = 1.325 ± 0.007 M_⊙, R_A = 2.525 ± 0.011 R_⊙ and R_B = 1.331 ± 0.006 R_⊙. We determine the abundance of 20 elements for the primary star, of which all except scandium are supersolar by up to 1 dex. The temperature of this star (9520 ± 120 K) makes it one of the hottest Am stars. We also measure the abundances of 25 elements for its companion (T_eff = 6880 ± 240 K), finding all to be solar or slightly above solar. The photospheric abundances of the secondary star should be representative of the bulk composition of both stars. Theoretical stellar evolutionary models are unable to match these properties: the masses, radii and temperatures imply a half-solar chemical composition (Z = 0.009 ± 0.003) and an age of 490-550 Myr. YZ Cas therefore presents a challenge to stellar evolutionary theory.